This invention relates generally to hydrostatic transmission drives. The term hydrostatic transmission is usually associated with highly developed and highly specialized components of closed loop systems, principally intended for precise control of a single load. In a closed loop system, fluid at a very high rate of flow circulates from the pump to the motor and back to the pump again, with only a comparatively small volume of oil being diverted from this closed loop for cooling purposes. Since in such a system any contamination in the loop will produce wear which in turn will produce more contamination, only the types of units generating the least amount of contamination have been successfully used, these having been piston-type pumps and motors, working in a pressurized circuit with a number of specialized accessories.
There is a large family of hydraulic motors of the non-piston type, which either due to their unique operating characteristics, low cost, or some other special features, are produced in very large quantities and used in open loop type hydrostatic transmission circuit, usually with a fixed displacement type pump. The control of the load in such a system is accomplished by means of throttling type direction control valves. Although usually the load control in such systems is not precise and the systems are inefficient, they are inexpensive and lend themselves to control not only of a single load, but a number of loads. The open loop system is also widely used for another, less obvious reason. Some of these high torque, low speed orbit-type hydraulic motors, some of the vane motors, and vane and gear pumps shed contaminants at a comparatively high rate. In an open loop circuit the full quantity of fluid is introduced into the pump from the reservoir, through suction filter or strainer, supplied to the motor and delivered from the motor back directly to the reservoir, usually through a return line filter. In this way, not only is the contamination generated by the motor never introduced directly into the pump inlet, but also contamination generated by the pump can pass only once through the motor. A mixed loop hydrostatic transmission system, according to this invention, retains the contamination free characteristics associated with the open loop circuit while retaining precise control of the closed loop hydrostatic transmission system.